Paper No.: p074
Laser Surface Alloying of Austenitic Stainless Steel With Cr and Ni for Enhanced Corrosion Resistance Author's Corrections:
Received on: Mailed back on:
R. Kaul
Initial
Industrial CO2 Laser Section, Centre for Advanced Technology Indore – 452 013 India E-mail:
[email protected]
U. Kamachi Mudali Corrosion Science & Technology Division, Indira Gandhi Centre for Atomic Research Kalpakkam – 603 102 Tamil Nadu, India E-mail:
[email protected]
P. Ganesh Industrial CO2 Laser Section, Centre for Advanced Technology Indore – 452 013 India
R. Jagdeesh Regional Engineering College, Tiruchirapalli – 620 012 Tamil Nadu, India
A.K. Singh and M.K. Tiwari Synchrotron Utilisation Division, Centre for Advanced Technology Indore – 452 013 India
V.S. Raju National Centre for Compositional Characterisation of Materials Hyderabad – 500 062 Andhra Pradesh, India
H.S. Khatak Corrosion Science & Technology Division, Indira Gandhi Centre for Atomic Research Kalpakkam – 603 102 Tamil Nadu, India
A.K. Nath Industrial CO2 Laser Section, Centre for Advanced Technology Indore – 452 013 India
International Conference on Advances in Surface Treatment: Research & Applications (SMT XVII & IFHTSE) EDITED BY T.S. S UDARSHAN, S.V. JOSHI and G. TOTTEN Copyright © 2003 by Society for Advancement of Heat Treatment & Surface Engineering (SAHTSE), c/o ARCI, Hyderabad, India
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Abstract Austenitic stainless steels are susceptible to pitting and crevice corrosion in chloride ion environment. The present study has aimed at enhancing pitting and crevice corrosion resistance of austenitic stainless steel by laser surface alloying with Cr and Cr and Ni. Surface alloying was effected by laser cladding type 304L stainless steel substrate with premixed powders of AISI 316 L stainless steel and the desired alloying elements. Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of about 24 wt.% whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of about 22 wt.% each. Such microstructural changes are expected to yield good corrosion resistance.
1.0 Introduction Austenitic stainless steels, because of their excellent mechanical properties, weldability and formability, find wide-ranging engineering applications. However, this class of stainless steel is susceptible to pitting and crevice corrosion in chloride bearing environment.1 Increase in chromium content is known to enhance corrosion resistance of austenitic stainless steel. Super austenitic stainless with Cr content of 25% (and Mo ≈ 6.5%) exhibit superior corrosion resistance than the popular 18/8 variety of stainless steel. Surface alloying of austenitic stainless steel with Cr, therefore, offers an economical means of enhancing corrosion resistance of relatively cheaper 18/8 austenitic stainless steel. High power lasers are emerging as an efficient tool to modify surfaces of engineering components. The present study was undertaken to enhance corrosion resistance of AISI 304 stainless steel austenitic stainless steel by effecting laser surface alloying with Cr and Ni. Laser surface alloying is usually achieved by surface melting of suitably coated substrate specimens (by electroplating, thermal spray or with the help of a suitable binder). The process, in addition to being more time consuming (multi-step process involves deposition of layers of different alloying elements followed by laser surface melting), usually yields an inhomogeneous-alloyed layer whose chemical composition is difficult to control. The process is also not suitable for effecting large change in chemical composition. In contrast, uniform surface layers of desired chemical composition can be efficiently deposited by laser surface cladding. In the present study, the process of laser cladding was used to suitably modify surface chemical composition of AISI 304 stainless steel substrate by increasing its Cr content. Since addition of Cr (a ferrite stabilizer) is expected to promote formation of delta ferrite, surface alloying was also carried out with Cr and Ni to control the amount of delta ferrite in the laser surface alloyed layer. Moreover, the addition of Ni along with Cr is known to improve oxide forming ability of Cr and increases corrosion resistance in more aggressive environments. 2, 3 Nickel is effective in promoting repassivation, specially in reducing environments and mineral acids.4 It also enhances resistance against stress corrosion cracking.5
2.0 Experimental Procedure Laser surface treatment experiments were carried out with an indigenously developed 2.5 kW continuous wave carbon
dioxide laser.6 The experimental set up consisted of a CO2 laser system, a beam delivery system and a computer controlled workstation. Laser beam, extracted from the laser system, was bent by a 45° mirror and was subsequently focused with the help of 120 mm focal length zinc selenide lens. Focusing lens was housed in a copper nozzle through which a shroud gas (argon) was flown during the course of laser treatment. The shroud gas not only maintained a shroud cover over laser-irradiated zone but also served to protect the expensive focusing lens from possible particulate emission from the surface being treated. Laser surface alloying was obtained by cladding AISI 304 L stainless steel substrate with premixed powders of AISI 316 L stainless steel and the desired alloying elements. The process involved scanning the surface of 6 mm thick substrate with a defocused laser beam of 1.6 mm diameter while simultaneously injecting premixed powder into laser-irradiated zone on the surface of the substrate. Particle size of the powders used for laser cladding was in the range of 50–100 µm. Overlapping tracks were laid to cover a larger surface area. In order to overcome dilution from the base metal, two layers of multi-track deposits were overlaid on the substrate. The details of the experimental parameters are summarised in Table 1. Laser treated specimens were subsequently characterised by quantitative energy dispersive X-ray fluorescence (EDXRF), optical microscopy, micro-hardness measurement, Energy-dispersive spectroscopy (EDS) in conjunction with scanning electron microscopy (SEM) and X-ray diffraction (XRD).
3.0 Results Low magnification macroscopic examination of laser treated specimens did not reveal any cracks. 3.1 Quantitative EDXRF Spectrometry In order to determine average surface chemical composition of laser-alloyed specimen, the top surface of the specimens were examined by quantitative EDXRF spectrometry. The examination was performed with an in-house developed EDXRF spectrometer.7 A 20 mCi Cd-109 radio-isotope was used as an excitation source. The detection system for energy dispersive measurement consisted of a Si(Li) detector having energy resolution of 150 eV at 5.9 keV, coupled to a spectrometer amplifier and a multi-channel pulse height analyzer. The net intensities of various fluorescence peaks
Laser Surface Alloying of Austenitic Stainless Steel with Cr and Ni for Enhanced Corrosion Resistance
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Table 1. Experimental Parameters
Specimen
Powder
Targeted Composition (wt%)
Laser Power (kW)
Scan Speed (mm/s)
Beam Dia (mm)
Powder Feed Rate (gm/min)
Degree of Overlap
Cr-Alloyed
316 L SS + Cr
Cr: 25–30; Ni: 8–10
1.5
5
1.6
5-6
40%
Cr + Ni Alloyed
316 L SS + Cr + Ni
1.5
5
1.6
5-6
40%
Cr: 25–30 Ni: 25–30
Table 2. Chemical Composition (in wt.%), as Determined by Quantitative EDXRF Spectrometry Specimen
Cr
Ni
Mn
Mo
Fe
Substrate
19.48
10.68
1.08
1566 ppm
67.99
Cr Alloyed
24.23
9.04
0.63
2.06
64.05
(Cr + Ni) Alloyed
24.42
21.72
0.27
1.37
52.22
were derived with spectrum processing software and nonlinear test square fitting procedure. Quantitative analysis was performed by using quantitative analysis program, CATXRF.8 Chemical composition (in wt.%) of the substrate and laser alloyed surfaces, as obtained from quantitative EDXRF spectrometry are presented in Table 2. It should be noted that the technique determines elemental concentration for medium and high atomic number elements (atomic number Z ≥ 17). In spite of this limitation, the results of EDXRF were good enough to predict phases present in the surface alloyed layer.
specimens, (Cr + Ni) alloyed specimens did not exhibit any significant difference in microstructure across its thickness. Both the clad layers were associated with primary austenite mode of solidification. Figures 5 and 6 present high magnification view of the microstructures of the bottom and top clad layers, respectively. The microstructure of the top clad layer indicates that the chemical composition of the clad layer fell in the eutectic triangle. Concurrent rise in Ni content along with that of Cr in (Cr + Ni) alloyed layer was responsible for offsetting ferrite-stabilizing effect of Cr addition to develop largely austenitic microstructure.
3.2 Metallographic Examination 3.3 Micro-Hardness Measurement Metallographic examination was performed on the polished and etched transverse cross-section of laser treated specimens.
3.2.1 Cr Alloyed Specimen Figure 1 presents the cross-section of one of the Cr alloyed AISI 304 L stainless steel specimens. The two clad layers exhibited distinctly different microstructures. The bottom-clad layer was associated with vermicular ferrite microstructure, representing primary ferrite mode of solidification. On the other hand, the top clad layer exhibited large amount of long acicular ferrite. Figures 2 and 3 present high magnification photomicrographs of the microstructures of bottom and top clad layers, respectively. Significant rise in ferrite content of the top clad layer over the bottom layer is indicative of relatively higher ferrite potential of the top layer because of reduced dilution from the substrate.
3.2.2 (Cr + Ni) Alloyed Specimen Figure 4 presents the cross-section of a (Cr + Ni) alloyed AISI 304 stainless steel specimen. In contrast to Cr alloyed
Micro-hardness measurements were carried out on the polished cross-sections of laser treated specimens with a load of 0.981 N (indentation duration = 30 seconds). Cr surface alloyed layer exhibited a minor rise in micro-hardness over the base metal. On the other hand, no significant change in microhardness was noticed in (Cr + Ni) alloyed specimen. Figure 7 presents micro-hardness profiles of Cr and (Cr + Ni) surface alloyed specimens. 3.4 SEM-EDS Examination In order to determine chemical composition gradient from substrate/clad interface to the surface, the cross-sections of laser treated specimens were analysed by SEM-EDS. In Cr alloyed specimen, Cr content registered an increase from about 19 wt.% in the base metal to about 23 wt.% at the surface. This was accompanied by relatively minor drop in the concentrations of Fe and Ni. Figure 8 presents concentration profiles of Cr, Ni and Fe (in wt.%) across Cr-alloyed layers. On the other hand, (Cr + Ni) surface alloyed specimen exhibited substantial rise in Ni concentration along with build
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Fig. 4:
Cross-section of Cr and Ni surface alloyed stainless steel specimen.
Fig. 1:
Cross-section of Cr surface alloyed stainless steel specimen.
Fig. 2:
Microstructure of the bottom layer of Cr surface alloyed stainless steel specimen.
Fig. 5:
Microstructure of the bottom layer of Cr and Ni surface alloyed stainless steel specimen.
Fig. 3:
Microstructure of the top layer of Cr surface alloyed stainless steel specimen.
Fig. 6:
Microstructure of the top layer of Cr and Ni surface alloyed stainless steel specimen.
Fig. 7:
600 400 200 0 200 400 600 800 1000 1200 1400 300 290 ¾ Cr Surface Alloyed Specimen 280 ¾ Cr & Ni Surface Alloyed Specimen 270 ¾ ¾ ¾ 260 ¾ ¾ ¾ ¾ 250 ¾ ¾ ¾ ¾ ¾¾ 240 ¾ ¾ ¾ ¾¾ ¾ ¾ F F 230 F ¾ F F ¾ F 220 F ¾ F¾ F F F F 210 F F 200 F F 190 180 Base metal Surface alloyed layer 170 -600 -400 -200 0 200 400 600 800 1000 1200 1400 Distance from fusion line, micrometer F
Microhardness, VHN (Load = 0.981 N)
Laser Surface Alloying of Austenitic Stainless Steel with Cr and Ni for Enhanced Corrosion Resistance
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300 290 280 270 260 250 240 230 220 210 200 190 180 170
Micro-hardness profiles across the cross-section of laser surface alloyed layers.
70
¾
¾
¾
Concentration, wt.%
60
¾
¾
¾
50 40
¾
Fe
Ni
Cr
30 20
10
Laser Surface Alloyed Layer
Base Metal 0 -200
0
200
400
600
800
1000
Distance from Fusion Line, Micrometer Fig. 8:
EDAX concentration profiles of Fe, Cr and Ni across the cross-section of Cr surface alloyed stainless steel specimen.
up of Cr concentration. Cr content of the surface alloyed layer rose from about 18 wt.% in the base metal to about 22% at the surface whereas Ni concentration increased from about 10% in the base metal to about 22 wt.% at the surface. Figure 9 presents concentration profiles of Cr, Ni and Fe (in wt.%) across (Cr + Ni) alloyed layers.
3.4 X-Ray Diffraction (XRD) In order to determine phases present on the top surface of laser treated specimens, the specimens were analysed by X-ray diffraction using CuKα characteristic radiation. Cr surface alloyed specimens exhibited the presence of austenite and ferrite
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¾
¾ ¾
60 Concentration, wt %
¾
40
¾
20
0
Fe
Ni
Cr
Laser Surface Alloyed Layer
Base Metal -200
¾
¾
30
10
Fig. 9:
¾
50
0
200 400 600 800 1000 Distance from Fusion Line, Micrometer
1200
EDAX concentration profiles of Fe, Cr and Ni across the cross-section of Cr and Ni surface alloyed stainless steel specimen.
1. γ–Fe (111) 2. α–Fe (110) 3. γ–Fe (200) 4. α–Fe (220) 5. γ–Fe (220)
300 12
4
3
1. γ-Fe (111) 2. γ- Fe (200) 3. γ−Fe (220)
200 1 5
2
3
100
150
0
0 23
43
2θ°
63
83
23
43
63
83
2θ°
Fig. 10: X-ray diffraction pattern of Cr surface alloyed stainless steel specimen.
Fig.11: X-ray diffraction pattern of Cr and Ni surface alloyed stainless steel specimen.
while (Cr + Ni) alloyed specimen was found to be associated with a single phase austenitic structure. Figures 10 and 11 present XRD patterns of Cr and (Cr + Ni) alloyed specimens.
registered a significant increase from 310 to 720 mV. Surface alloying with both Cr and Ni served to further raise pitting potential to 980 mV. Figure 12 compares potentiodynamic polarisation curves of laser treated specimens with that of the substrate. Impedance tests performed on laser treated and AISI 304 L stainless steel substrate specimens showed that Cr+Ni alloyed specimen had highest polarisation resistance followed by Cr-alloyed specimen as compared to untreated substrate. The capacitance of Cr + Ni alloyed specimen was lowest followed by Cr-alloyed specimen and untreated substrate. Figure 13 presents results of the impedance study.
3.5 Corrosion Testing Potentiodynamic polarisation study was performed on laser surface treated specimens as well as on AISI 304 L stainless steel substrate in 0.5 M NaCl solution. Preliminary results showed excellent beneficial effect of laser surface alloying with Cr and Cr + Ni. Pitting potential of Cr alloyed specimen
Laser Surface Alloying of Austenitic Stainless Steel with Cr and Ni for Enhanced Corrosion Resistance 1200
304L 30C r2 30C r N i
Potential, mV vs Ag/AgCl
1000 800 600 400 200 0 -200 -400 -600 1E-3
0.01
0.1
1
10
100
1000
10000
Current Density , µ A/cm 2
Fig. 12: Potentiodynamic polarisation curves of laser treated specimens and the substrate in 0.5M NaCl solution (30Cr2-Cr alloyed specimen, 30CrNi - Cr + Ni alloyed specimen).
304 L 30Cr2 30CrNi
C , F/cm2
1.5 × 10-5
1.0 × 10-5
○
5.0 × 10-6
○
○
○
○
○
0
○
○
○
○
○
○
500
○
○
○
○
○
○
1000
○
○
○
○
1500
Time, Sec
30
304 L
R, ohms cm2
20 ○
○
○
○
○
30Cr2 30CrNi
○
○
○
○
○
○
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○
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○
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○
○
○
○
10
0
0
500
1000
1500
Time, Sec
Fig. 13: Impedance parameters of capacitance and polarisation resistance behavior (30Cr2 - Cr alloyed specimen, 30CrNi - Cr + Ni alloyed specimen).
4.0 Discussion Austenitic stainless steels derive their corrosion resistance from the presence of a protective and self-healing chromium oxide layer on their surface. However, these stainless steels are susceptible to localised corrosion under chloride ion environment. The sites of chloride ions adsorption on the surface of stainless steel (preferably at non-metallic inclusion/
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steel interface, carbides, grain boundaries and other microstructural in-homogeneities where passive oxide film is defective) act as pre-nuclei of pits. 9 Increase in Cr concentration of the austenitic stainless steel is known to enhance its resistance against localised corrosion.10 Since increase in Cr concentration can lead to the development of substantial amount of ferrite, a concurrent addition of Ni was also required to maintain the surface in essentially austenitic condition. Khanna and Sridhar reported development of super austenitic stainless layer on AISI 304 stainless steel substrate by laser surface alloying involving melting of thermally sprayed layers of Ni-Cr and Mo.11, 12 However, the chemical composition and corrosion resistance of the resultant surface alloyed layer was found to be critically dependent on laser processing parameters. Except for one specimen, which had highest Mo content of 12.6 wt.% (and Cr ≈ 17.9 wt.%), others exhibited either a very narrow passive plateau or its absence. Specimens even with Mo content of more than 6 wt.% (and Cr ≈ 16–19 wt.%) did not exhibit a passive plateau. Authors concluded that the associated compositional fluctuation in laser surface alloyed layers facilitated the formation of local galvanic cells leading to the breakdown of passive films. In contrast, to the process of laser melting of pre-coated specimens, laser cladding process appears to have a greater control over the chemical composition of the resultant surface layer. The resultant surface alloyed specimens exhibited uniform microstructure parallel to the top surface. Microstructural phases present in the laser surface alloyed austenitic stainless steel can be accurately predicted from the wellknown Schaffler diagram.13 Creq and Nieq values, estimated from EDXRF results (assuming C content to remain unaltered), predicted a duplex (austenite +30% ferrite) microstructure in the top Cr alloyed layer while a completely austenitic microstructure was expected in the top (Cr + Ni) alloyed layer. The predictions made from the Schaffler diagram were further supported by X-ray diffraction and micro-structural analysis. Micro-structural transition from single phase austenite to duplex (austenite + ferrite) in Cr alloyed specimens was accompanied by a small increase in micro-hardness. On the other hand, (Cr + Ni) surface alloyed specimen that did not experience any significant microstructural transition exhibited largely uniform micro-hardness across the cross-section of laser-treated specimen. Preliminary corrosion results carried out in 0.5 M NaCl solution revealed that the pitting resistance of austenitic stainless steel surface is effectively enhanced by laser surface alloying with Cr and Ni. The rise in pitting potential was more than three fold. The combined effect of Cr and Ni surface alloying in raising pitting potential was more pronounced than that induced by surface alloying with Cr alone. Results of impedance tests indicate that the superior corrosion resistance of laser surface alloyed specimens is attributed to the presence of thicker and more protective passive film than that present on the surface of 304 L stainless steel substrate.
5.0 Conclusion The present study was successful to effectively control chemical composition of AISI 304 stainless steel, close to
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Kaul et al.
the targeted value. Preliminary corrosion results showed significant improvement in pitting resistance of austenitic stainless steel as a result of laser surface alloying with Cr and Ni.
8.
6.0 Acknowledgment Technical assistance of Mr. CH Prem Singh and Mr. Ram Nihal are thankfully acknowledged.
7.0 References 1.
2. 3. 4. 5. 6. 7.
U. Kamachi Mudali and M.G. Pujar, Corrosion of Austenitic Stainless Steels, H.S. Khatak and Baldev Raj, eds., Narosa Publishing House, New Delhi, 2002, pp.74–105. Web site: www.adworkdesignfirm.com/clients/csw/ stainless.html, 2003. Web site: www.hazak.co.uk/frame/steel/F.htm# resistance, 2003. ASM Metals Handbook, Corrosion, 9th edition, ASM International, Ohio, 13, 1992, pp.550–559. Web site: www.stainlessproducts.com/p47.shtml, 2003. A.K. Nath, L. Abhinandan and P. Choudhary, Optical Engineering, 33(6), 1994, p.1889. K.J.S. Sawheney, M.K. Tewari, A.K. Singh and
9.
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13.
R.V. Nandedkar, Proceedings of Sixth National Seminar on X-ray Spectroscopy and Allied Areas, S.K. Joshi et al., eds., 1998, pp.130–133. M.K. Tewari and K.J.S. Sawheney, CATXRF: A Quantitative Analysis Program for Energy Dispersive X-ray Fluorescence (EDXRF) Spectrometry, CAT Technical Report, 2000. S. Szlarska-Smialowska, Nucleation of Localised Corrosion in Stainless Steel Exposed to Chloride Solution, Corrosion, V. Ashforth, ed., KFAS Proceeding Series, Pergamon, Books Ltd. Oxford, 2, 1984, pp.29–43. B. Baroux, F. Dabosi and C. Lemaitre, Pitting Corrosion and Crevice Corrosion, In: Stainless Steels, P. Lacombe, B. Baroux and G. Beranger, eds., Les Editons de Physique Les Ulis, 1993, pp.778–781. A.S. Khanna and K. Sridhar, Corrosion and Oxidation Behaviour of Laser Treated Surfaces, Lasers in Surface Engineering, N.B. Dahotre, ed., Surface Engineering Series, ASM International, Ohio, 1, 1998, pp.413–422. K. Sridhar, A.S. Khanna, A. Gasser and M.B. Deshmukh, Laser Surface Alloying of Type 304 Stainless Steel for Enhanced Corrosion Resistance, Lasers in Engineering, 6, 1996, pp.107–125. Welding Handbook, Materials & Application, 8th edition, American Welding Society, Miami, FL, 4(2), 1998, pp.264–267.
